Howdy Bob:
Typically, on a mostly stock engine, you would use a ported vacuum source. The key here is vacuum. If you have a longer then stock cam that reduces vacuum at idle, then you would want to experiment with a manifold vacuum source.
The biggest difference is what happens to your vacuum advance at idle. Ported vacuum source= little or no advance caused by vacuum. Manifold vacuum source= relatively high vacuum advance.
You might need to do some experimenting to see what works best for you. It is a balance between the initial advance setting and vacuum advance. No two situations are the same.
Check it out and let us know what you find.
Adios, David
